Zirconium-doped pseudobehmite, process for its production and use thereof

ABSTRACT

Zirconium-doped pseudoboehmite with the following properties: zirconium content of 0.5 to 10 mol % relative to Al water content of 20 to 35 weight percent total of the anionic impurities less than 0.5 weight percent. It is synthesized by a process wherein zirconium salt is added in calculated quantity to an aqueous aluminum sulfate solution, the resulting mixture is fed simultaneously with a sodium aluminate liquor into a water pool having a temperature of 50° to 100° C., thereby precipitating zirconium-doped alumina gel while a pH of 3.5 to 8 is maintained, the pH of the suspension is adjusted after addition of the aluminum sulfate solution to 8.5 to 10.5 by further addition of sodium aluminate liquor, the suspension is digested if necessary at the pH of 8.5 to 10.5 and the temperature of 50° to 100° C. for a period of up to 24 hours, the formed pseudoboehmite crystals are filtered off, the filter cake is redispersed with water and the suspension is sprayed-dried. By calcining at 700° to 1100° C., preferably at 850° to 950° C., the pseudoboehmite is further processed to zirconium-doped gamma-alumina suitable for automobile exhaust catalysts.

INTRODUCTION AND BACKGROUND

The present invention relates to a zirconium-doped pseudoboehmite and the process for its production. In addition, the invention relates to zirconium-doped gamma-alumina synthesized from the pseudoboehmite, the process for its production as well as its use for making automobile exhaust-gas catalysts.

It is known that hydrated alumina containing essentially pseudoboehmite can be synthesized (European Patent 113,796).

Pseudoboehmites can be used for making automobile exhaust-gas catalysts.

It is also known that, in order to make a support catalyst for purification the exhaust gases of internal combustion engines, a ceramic honeycomb body can be immersed in an aqueous suspension which contains a mixture of pseudoboehmite and zirconium oxyhydrate (West German Patent 3,539,127).

In the automobile exhaust-gas catalysts of known composition, sintering of the basic material, and thus deactivation by loss of surface area, can occur due to the high operating temperatures.

SUMMARY OF THE INVENTION

An object of the invention is to produce a support material which exhibits, at high operating temperatures, little shrinkage or little surface loss and, associated therewith, less deactivation.

In achieving the above and other objects, one feature of the invention is a zirconium-doped pseudoboehmite, which has the following properties: zirconium content: 0.5 to 10 mol % relative to Al; water content: 20 to 35 weight percent total of the anionic impurities: <0.5 weight percent.

The zirconium-doped pseudoboehmite incorporating the invention can be produced in a process wherein a calculated amount of zirconium salt is added to an aqueous aluminum sulfate solution, the resulting mixture is fed simultaneously with a sodium aluminate liquor into a pool of water having a temperature of 50° to 100° C., thereby precipitating zirconium-doped alumina gel while maintaining a pH of 3.5 to 8. The pH of the suspension is adjusted after addition of the aluminum sulfate solution to 8.5 to 10.5 by further addition of sodium aluminate liquor. The suspension is digested if necessary at the pH of 8.5 to 10.5 and at a temperature of 50° to 100° C. for a period of up to 24 hours. The formed pseudoboehmite product is filtered off and the filter cake is washed with water.

In another embodiment of the invention, the zirconium salt is dissolved in water and this solution is added to the pool of water simultaneously with the aluminum sulfate solution and the sodium aluminate liquor.

In a special embodiment of the invention, the washed filter cake is redispersed in water and the resulting suspension is spray-dried.

In one preferred embodiment of the invention, the precipitation is conducted at a temperature of 60° to 80° C. and a pH of 4 to 5.

Preferably, the digestion is carried out at a temperature from 60° to 80° C. and a pH from 9 to 9.5 for a period of one hour to 5 hours.

The spray-dried, zirconium doped pseudoboehmite can be calcined at a temperature of 700° to 1100° C., preferably 850° to 950° C., for a period of 0.5 to 24 hours. In that way the pseudoboehmite is converted into zirconium-doped gamma-Al₂ O₃, which has the desired specific BET surface of 145±25 to 225±25 m² /g.

The Zirconium component can be used in the form of salts, e.g. zirconium chloride, zirconium nitrate, zirconium carbonate.

Another feature of the invention is the zirconium-doped gamma-alumina synthesized from the zirconium-doped pseudoboehmite according to the invention. This gamma-alumina has a zirconium content of 0.5 to 10 mol % relative to Al, a water content of <10 weight percent and a phase stability up to 1050° C. The temperature stability of the gamma-alumina is significantly improved by doping with zirconium.

The zirconium-doped gamma-alumina according to the invention can be used for making automobile exhaust-gas catalysts.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described more fully with reference to the accompanying drawings, in which:

FIG. 1 shows a flow diagram of the process according to the invention for the production of zirconium-doped pseudoboehmite.

FIG. 2 shows the influence of the doping level on the pseudoboehmite formation.

FIG. 3 shows the surface loss as a function of calcining duration at 900° C.

FIG. 4 shows the surface loss as a function of calcining temperature (1-hour holding time).

FIG. 5 shows the phase analysis after calcining at 1050° C. (24 hours holding time).

FIG. 6 shows the phase analysis after annealing at 1250° C. (1-hour holding time).

DETAILED DESCRIPTION OF EMBODIMENTS Examples

The examples are carried out by the process described schematically in FIG. 1.

As is generally known for pseudoboehmite formation, the most relevant reaction parameters are the pH and temperature conditions of the precipitation and the digestion process. The examples illustrate the influence of the doping material and of the doping level (Zr/Al molar ratio) on the surface stabilization and on the crystallographic formation as well as chemical purity; the results are summarized in Tables 1 to 3, and shown in FIG. 2. Selected samples are examined in detail with respect to their phase and surface stability, and compared to known commercial products (Kaiser SA washcoat grade, Pural SB and Pural SF of the CONDEA) in FIGS. 3 to 6.

The samples are examined immediately after pH adjustment and after a 20- to 23- hour digestion. Thereby, it is found that even small additions of doping material can strongly influence the pseudoboehmite formation.

Additions of zirconium chloride in a molar ratio of Zr/Al=1/100 (Table 1) and Zr/Al=5/100 (Table 2) lead to no fundamental change in pseudoboehmite crystallization. In a manner similar to the case of precipitation of pure pseudoboehmite, alumina trihydrate can be formed in different concentrations when the precipitation is conducted at pH=6 to 8.

The gradual influence of doping with zirconium chloride on the crystallization at a precipitation pH of 4 is presented in Table 3 and illustrated in FIG. 2: with increasing zirconium concentration, either the product becomes more amorphous or trihydrate forms as a secondary phase and disappears only during digestion.

Additions of zirconium nitrate or carbonate cause a shift toward formation of alumina trihydrate or amorphous substances (Table 2).

For the production of pure pseudoboehmite, it is necessary to adjust the pH of the suspension within a narrow range around the isoelectric point, in order to minimize the adsorption of sodium and sulfate ions.

By doping with zirconium chloride, however, it is possible to improve the chemical purity of the product to a great extent, since the formation of insoluble sulfate (in the form of basic alumina) is suppressed or reduced (Tables 1 to 3).

The determination of the specific surface is carried out on digested samples, as obtained in fresh form after spray drying or after they have been calcined for one hour at 900° C. The results are listed in Tables 1 to 3.

The data show that the increase of the specific surface area is achieved by optimizing the precipitation and digestion conditions for specific doping components.

Zirconium-doped pseudoboehmite samples are calcined under different conditions and compared with identically treated commercial products: FIGS. 3 and 4 respectively clearly show the superiority of the zirconium-doped pseudoboehmite and gamma-aluminas according to the invention.

The X-ray diagrams of the calcined samples (FIGS. 5 and 6) provide a plausible explanation for the surface stability of the doped test products:

In the case of Kaiser SA, the transformation to alpha-alumina is advanced at 1050° C. and is complete at 1250° C.

No improvement can be seen by admixing zirconium carbonate.

In the case of zirconium-doped pseudoboehmite, the gamma phase is practically unchanged at 1050° C. and is still largely preserved up to 1250° C.

By precipitation with transition-metal oxides, doped pseudoboehmites having high specific surfaces are obtained under optimized conditions. Because of the doping, the deactivation of the alumina under thermal loads is greatly retarded, and so the new materials are very suitable as catalyst supports for use at very high temperatures (about 1000° C.).

Further variations and modifications will be apparent to those skilled in the art from the foregoing and are intended to be encompassed by the appended claims. German priority application P. 38 01 270.7-41 is relied on and incorporated herein.

                                      TABLE 1                                      __________________________________________________________________________     Influence of the precipitation and digestion: Doping in Zr/Al molar            ratio = 1/100                                                                  Precipita-             Specific                                                tion       Digestion   surface                                                                               Chemical analysis                                                                        Phase analysis*                        Temp.      Temp.  Duration                                                                            Fresh                                                                              Calc.                                                                             Na.sub.2 O                                                                          SO.sub.4                                                                            a    b                                 Doping                                                                              (°C.)                                                                       pH                                                                               (°C.)                                                                       pH (h)  (m.sup.2 /g)                                                                          (ppm)                                                                               (wt %)                                                                              PB/TH                                                                               PB/TH                             __________________________________________________________________________     none 60  4 80  9  20   268 154                                                                               30   2.07 m    m                                 ZrCl.sub.4                                                                          60  4 60  9  21   294 159                                                                               28   1.91 w    m                                      80  4 80  9   1   304    80   0.11      m                                      80  4 80  9  20   308 204                                                                               77   <0.1 m    m                                      60  4 80  9.5                                                                               21   243 151                                                                               73   <0.05                                                                               m    m                                      80  4 80  9.5                                                                                1   335 183                                                                               87   <0.02     m                                      60  6 60  9.5                                                                               23   285 166                                                                               30   <0.1 m/vs m/t                                    60  6 80  9.5                                                                               22   282 171                                                                               75   <0.1 w    m                                      80  6 80  9.5                                                                               21   345 145                                                                               290  <0.1 m/vs m                                      60  8 60  10 21   300 189                                                                               65   <0.1 m/t  m                                      60  8 80  9.5                                                                               22   306 190                                                                               89   <0.1 w    m                                 Zr(CO.sub.3).sub.2                                                                  60  4 80  9.5                                                                               22   272 148                                                                               94   <0.1 w    m                                 Zr(SO.sub.4).sub.2                                                                  80  4 80  9.0                                                                               21   not determined**                                             80  4 80  9.5                                                                               21   not determined**                                        __________________________________________________________________________      *a = product was isolated and examined before digestion                        b = product examination after the indicated digestion time                     PB = pseudoboehmite                                                            TH = trihydrate                                                                t = traces                                                                     w = weak                                                                       m = medium                                                                     vs = very strong                                                               **smeary product which was difficult to filter, and which was not further      examined                                                                 

                                      TABLE 2                                      __________________________________________________________________________     Influence of anions: Doping with ZrCl.sub.4, Zr(NO.sub.3).sub.2                Zr(CO.sub.3).sub.2 ;                                                           Zr/Al molar ratio = 5/100                                                      Precipita-              Specific                                               tion        Digestion   surface                                                                               Chemical analysis                                                                        Phase analysis*                       Temp.       Temp.  Duration                                                                            Fresh                                                                              Calc.                                                                             Na.sub.2 O                                                                          SO.sub.4                                                                            a    b                                Doping                                                                               (°C.)                                                                       pH                                                                               (°C.)                                                                       pH (h)  (m.sup.2 /g)                                                                          (ppm)                                                                               (wt %)                                                                              PB/TH                                                                               PB/TH                            __________________________________________________________________________     ZrCl.sub.4                                                                           60  4 60  9.5                                                                               21   276 163                                                                                36  <0.1 w    w                                      60  4 80  9.5                                                                               22   267 157                                                                                79  <0.1 m    m                                      80  4 80  9.5                                                                                1   345 207                                                                                87  <0.1      w                                      80  4 80  9.5                                                                               22   304 192                                                                               161  <0.1 w    m                                      60  6 60  9.5                                                                               22   310 173                                                                                93  <0.1 w    w/t                                    60  6 80  9.5                                                                               22   306 164                                                                               138  <0.1 w/vs m                                      60  8 60  9.5                                                                               22   322 201                                                                               152  <0.1 w/vs m/m                                    60  8 80  9.5                                                                               22   375 203                                                                               132  <0.1 w/s  m                                      80  8 80  10 22   301 177                                                                               212  <0.1 m/s  m                                      80  8 80  10.5                                                                              22   246 152                                                                               2000 <0.1 m/s  m/vs                             Zr(NO.sub.3).sub.4                                                                   60  4 80  9.5                                                                               21   352 225                                                                               143  <0.1 w/vs                                        80  6 80  9.5                                                                               22   299 174                                                                               148  <0.1 w    w                                      80  8 80  9.5                                                                               22   296 184                                                                               225  <0.1 w    w                                Zr(CO.sub.3).sub.2                                                                   60  4 80  9.5                                                                               22   296 184                                                                               225  <0.1 m/vs m                                __________________________________________________________________________      *a = product was isolated and examined before digestion                        b = product examination after the indicated digestion time                     PB = pseudoboehmite                                                            TH = trihydrate                                                                T = traces                                                                     w = weak                                                                       m = medium                                                                     s = strong                                                                     vs = very strong                                                         

                                      TABLE 3                                      __________________________________________________________________________     Influence of doping level: Doping with ZrCl.sub.4                                      Precipita-        Specific                                                     tion  Digestion   surface                                                                               Chemical analysis                                                                        Phase analysis*                             Temp. Temp.  Duration                                                                            Fresh                                                                              Calc.                                                                             Na.sub.2 O                                                                          SO.sub.4                                                                            a    b                              Doping  (°C.)                                                                       pH                                                                               (°C.)                                                                       pH (h)  (m.sup.2 /g)                                                                          (ppm)                                                                               (wt %)                                                                              PB/TH                                                                               PB/TH                          __________________________________________________________________________     0.5 Mol % Zr                                                                           80  4 80  9.5                                                                               1    330 185                                                                               30   3.2  --   m                                1 Mol % Zr                                                                           80  4 80  9.5                                                                               1    335 183                                                                               87   <0.02                                                                               --   m                                5 Mol % Zr                                                                           80  4 80  9.5                                                                               1    345 207                                                                               87   <0.1 --   w                               10 Mol % Zr                                                                           60  4 80  9.5                                                                               22   306 169                                                                               630  <0.1 w/vs w                              __________________________________________________________________________      ***a = product was isolated and examined before digestion                      b = product examination after the indicated digestion time                     PB =  pseudoboehmite                                                           TH = trihydrate                                                                A = amorphous product                                                          w = weak                                                                       m = medium                                                                     vs = very strong                                                          

We claim:
 1. A spray dried zirconium-doped pseudoboehmite comprising:a zirconium content of 0.5 to 10 mol percent relative to aluminum; a water content of 20 to 35 weight percent; and a total of anionic impurities of less than 0.5 weight percent.
 2. A zirconium-doped gamma-alumina comprising a zirconium content of 0.5 to 10 mol percent relative to aluminum, a water content of less than 10 weight percent, and a phase stability of up to 1050° C. and a BET surface of 145±25 m² /g to 225±25 m² /g.
 3. A zirconium-doped pseudoboehmite comprisinga zirconium content of 0.5 to 10 mol percent relative to aluminum; a water content of 20 to 35 weight percent; and a total of anionic impurities of less than 0.5 weight percent, said pseudoboehmite produced by the process comprising the steps of: adding a zirconium salt to an aqueous aluminum sulfate solution, feeding the resulting mixture simultaneously with sodium aluminate solution into a water pool having a temperature of 50° to 100° C. thereby precipitating zirconium-doped alumina gel while maintaining a pH of 3.5 to 8 in said pool to thereby form a suspension, adjusting the suspension having the addition of the aluminum sulfate solution to a pH of 8.5 to 10.5 by further addition of sodium aluminate, digesting the resulting suspension at a pH of 8.5 to 10.5 and a temperature of 50° to 100° C. for a period of up to 24 hours, filtering the pseudoboehmite formed thereby to obtain a filter cake, redispersing the filter cake with water to form a suspension and spray drying the resulting suspension.
 4. A zirconium-doped gamma-alumina comprising a zirconium content of 0.5 to 10 mol percent relative to aluminum, a water content of less than 10 weight percent, and a phase stability of up to 1050° C. and a BET surface of 145±25 m² /g to 225±25 m² /g,said gamma-alumina produced by the process comprising the steps of: adding a zirconium salt to an aqueous aluminum sulfate solution, feeding the resulting mixture simultaneously with sodium aluminate solution into a water pool having a temperature of 50° to 100° C. thereby precipitating zirconium-doped alumina gel while maintaining a pH of 3.5 to 8 in said pool to thereby form a suspension, adjusting the suspension having the addition of the aluminum sulfate solution to a pH of 8.5 to 10.5 by further addition of sodium aluminate, digesting the resulting suspensions at a pH of 8.5 to 10.5 and a temperature of 50° to 100° C. for a period of up to 24 hours, filtering the pseudoboehmite formed thereby to obtain a filter cake, redispersing the filter cake with water to form a suspension and spray drying the resulting suspension, and further calcining said pseudoboehmite at a temperature of 700° to 1100° C. to produce said zirconium-doped gamma-alumina. 